HLN is not naming the boys because the case is being tried in juvenile court.

The victim became a target of the teens' aggression after telling school administrators that some of his summer school classmates tried to sell him marijuana. Later that day, the four boys all got on the same bus home from school.

The victim's grandmother and legal guardian told CNN's "New Day" that the 13-year-old "should have never been put on that bus" after he told on the teens.

Gross also pointed out that the teens had almost 30 minutes from the time they got on the bus in Gulfport until the beating started to try and calm down. He says their rage only seemed to build during that time period.

Prosecutor William Schopper told Gross that investigators did not find any drugs on the boys. He also choked back tears after the video was played.

Two of the teens showed no emotion while watching the video, while the third teen nervously swiveled back and forth in his chair and looked down.

"I was angry. I felt disrespected, because something bad was said about me that wasn't true, and I took it in my own matters, and things just got worse," said the teenager who was referred to as the "ringleader" by Gross.

The judge asked the teen whether he had a message for the boy he attacked in his fit of rage.

"That I am sorry for doing it," the boy replied.

Another boy, whom Gross called a "follower," bent over and appeared to hyperventilate as he was sentenced.

Attorneys for the three accused teens told Gross their clients wrote sincere and heartfelt letters of apology to the victim and his family.

All three of the boys pleaded guilty to aggravated battery and one of the boys also pleaded guilty to robbery for stealing money during the beating.

The conditions of the teens' sentences include a period of electronic monitoring up to 60 days, community service and direct reporting to a probation officer. Probation for the three boys will continue until the court finds it appropriate to release them.

The 64-year-old bus driver, who was criticized for not stepping in to stop the fight, has since retired.

The boy's grandmother, who is also his legal guardian, told CNN's "New Day" that the teen was hit 29 times during the fight and suffered a broken hand.

The grandmother said if administrators had called her to pick up her grandson after he mentioned the alleged drug sale attempt, "This would not have happened."

"I have a big problem with that. I have a real big problem with that," she said. "I should have been called before something happened, not after something happened and been told that there was an incident."

When asked to comment, Pinellas County School District spokeswoman Melanie Marquez Parra said the district does not comment on specific incidents, noting that situations are handled on a case-by-case basis.

One of the boys' fathers previously offered an apology.

"I know I didn't raise him like that," he said. "I just feel like he made a bad decision and he just got caught up in a bad situation."

He suggested peer pressure may have played a role, but said he hoped his son had learned a lesson.

Earlier this month, bus driver Charles Moody told CNN's "Piers Morgan Live" that he was calling for help on his radio, as school policy required, but that jumping into the fight "would have been more dangerous for other students on the bus."

According to Pinellas County school policy, the bus driver isn't required to intervene, only to call dispatchers. He can step in, if he feels it's safe.